The field of the invention is systems and method for magnetic resonance imaging (“MRI”). More particularly, the invention relates to methods for producing motion-compensated MR images.
Magnetic resonance imaging (“MRI”) is highly sensitive to patient motion. Depending on the k-space acquisition trajectory, motion can cause blurring, ghosting, or other artifacts that reduce image quality and diagnostic value of the images, thereby reducing the diagnostic information available to the clinician or requiring repeated scans. Most physiological motion artifacts can be suppressed by proper gating techniques; however, bulk motion remains a clinical problem, particularly in three-dimensional imaging where the prolonged acquisition time increases the likelihood of the occurrence of motion artifacts. Especially challenging subject groups include pediatric, uncooperative, and impaired patients.
Gross body motion such as muscle twitching, adjusting for comfort, or restlessness often occurs in a discontinuous manner with longer, interleaved periods in which no motion occurs. Many successful retrospective motion correction techniques rely on this assumption and compare low resolution images that function as navigators acquired during the motion-free segments for successive motion correction. PROPELLER is one such approach, and has achieved widespread use in clinical applications, particularly in cranial imaging. In a PROPELLER scan, two-dimensional Cartesian k-space data are acquired in successively rotated strips of parallel lines called “blades,” with the assumption that motion occurs in-plane only and that no motion occurs during the acquisition of a blade. Motion parameters for translational and rotational in-plane motion are then determined by comparing low-resolution information from individual blades. Wider blades allow for more sensitive corrections; however, increasing the number of echoes per blade also increases the probability of in-blade motion and, for some pulse sequences, is limited by the available echo-train length. For the same reasons, straightforward extensions of PROPELLER to three dimensions by acquiring three-dimensional bricks instead of blades may have limited performance.
Despite reduction in imaging times through improved hardware and rapid acquisition schemes, motion artifacts can compromise image quality in MRI. This is especially true for three-dimensional imaging, where scan durations are prolonged and the assumptions of most state-of-the-art two-dimensional rigid body motion compensation techniques break down.
It would therefore be desirable to provide a method for producing motion-compensated images, especially in three dimensions, without the need for additional navigator data or external motion estimation schemes.